The carcinogen/mutagen metabolism capability of mammalian cells used in mutation and transformation assays is being increased by infecting the cells with retroviral vectors containing cDNAs coding for drug metabolizing enzymes. The cDNAs coding for the cytochromes P450 2A3, 2B1, 4B1, and a flavin monoxygenase have been inserted into vectors and infected into mouse embryo C3H/10T-(one-half) cells and hamster ovary AS52 cells. Western analysis and/or enzyme activities have demonstrated that the enzymes are expressed in the infected cells. During this last year, emphasis has been placed on cytochrome P450 2A3 expressing cells. C3H/10T-(one-half) cells expressing 2A3 activity have an increased cytotoxic sensitivity and are transformed to type III foci by the nitrosamines, NNK (a tobacco smoke component), and dimethyl- and diethylnitrosamine. The parental 10T-(one- half), cells are not subject to the cytotoxic or transforming effects of these chemicals. The 2A3 expressing 10T-(one-half), cells are also mutated to ouabain resistance by NNK. AS52 cells (in collaboration with Ken Tindall) expressing P450 2A3 have recently been obtained and mutation and mutational spectra studies are beginning. Overall, these studies continue to improve the usefulness of cultured mammalian cells to identify and study the modes of action of environmental chemicals.